This invention relates generally to aircraft electrical power supplies and, more specifically, to providing a more efficient source of electrical power for aircraft.
As is understood in the art, aircraft may use two types of auxiliary power units (APUs)xe2x80x94turbine-driven APUs and fuel cell APUsxe2x80x94to provide electricity to onboard systems. Turbine-driven APUs burn jet fuel to drive a turbine, which in turn drives a generator to produce electrical energy. Fuel cells chemically convert fuel into electrical energy. Both types of APUs may be used to power onboard systems while the aircraft is on the ground, as well as when the aircraft is in flight.
As useful as both types of APUs may be, however, each has disadvantages. Turbine-driven APUs may contribute appreciable pollution to an airport environment, because the burning of fuel to drive the turbines leaves some jet fuel uncombusted and produces oxides of nitrogen (NOX). Fuel cell APUs, on the other hand, offer lower emissions than turbine-driven APUs, but tend not to be efficient at high altitudes. In turbine-powered systems and fuel cels, attempts are made to exploit waste heat generated from the burning of combusted fuel by turbine-driven APUs or exothermically produced by fuel cell APUs to heat buildings or water for ground facilities.
In both cases, aircraft or other systems could benefit from improved APUs. More efficient fuel cell APUs, particularly at higher altitudes, would reduce the demand for other sources of electrical power.
Thus, there is an unmet need in the art for an improved APU and power generation method to cost-effectively provide sufficient electrical power while generating less pollution and consuming less fuel than current systems.
The present invention provides an apparatus and a method for improving the efficiency and energy output of a power unit. The present invention improves efficiency of a chemical process for converting combustible fuel into electrical energy by providing a supply of compressed air to the process. Combustible fuel not consumed by the chemical process is collected and burned to drive a turbine which, in turn, drives a compressor to supply the compressed air. In addition, an electrical generator can be coupled to the turbine to produce additional electrical energy. A starter motor also may be coupled with the compressor to compress air during a starting process.
An exemplary embodiment of the present invention is a power unit in which a fuel cell is configured to chemically convert combustible fuel into electrical energy. A waste fuel burner is configured to receive uncombusted fuel emitted by the fuel cell, and the waste burner combusts the uncombusted fuel to generate heated gas. A turbine is configured to receive and be driven by the heated gas. The turbine is further configured to drive a drive shaft which is coupled to a compressor. The compressor is configured to compress a pressurized source of oxidizing gas for supplying compressed oxidizing gas to the fuel cell such that efficiency of the fuel cell is improved.
The present invention may also incorporate a fuel reformer which can extract reactants from waste heat produced by the fuel cell. The present invention also may supply water to the reformer to facilitate the extraction of reactants. If desired, the water may be supplied by a water separator which extracts water from the heated gas. Water not used by the reformer may provide a water source for other purposes.
A form of the present invention may be used as an aircraft auxiliary power unit. The power unit suitably compresses air to provide a pressurized input gas to the fuel cell to enhance production of electrical power.